1. Field of the Invention
The present invention relates to a flexible circuit board whose entire surface has flexibility.
2. Description of the Related Art
A flexible circuit board made of a polyimide is known as a representative example of a flexible circuit board whose entire surface has flexibility. A flexible circuit board made of a polyimide is formed by stacking polyimide films, to each of which a metal film is attached, on top of one another and has an advantage in that it can be manufactured at extremely low cost. However, in a flexible circuit board made of a polyimide, accuracy of electrode shape and accuracy of the flexible circuit board shape are low, and a flexible circuit board made of a polyimide has a disadvantage in that consistently good electrical characteristics are difficult to realize.
Accordingly, a rigid-flexible circuit board only a portion of which has flexibility has sometimes been used (see, for example, Japanese Unexamined Patent Application Publication No. 2008-258357). A rigid-flexible circuit board has a configuration in which a rigid portion and a polyimide portion are connected to each other. In a rigid-flexible circuit board, a polyimide portion ensures flexibility, and a rigid portion ensures a highly accurate shape of an electrode and a highly accurate shape of the rigid-flexible circuit board, so that consistently good electrical characteristics can be realized in the rigid-flexible circuit board. However, such a rigid-flexible circuit board cannot be used in an application that requires a circuit board whose entire surface has flexibility, and it is difficult to reduce the thickness of the entire rigid-flexible circuit board.
Therefore, as a circuit board in which a highly accurate shape of an electrode and a highly accurate shape of a circuit board can be realized, a flexible circuit board made of a thermoplastic resin, such as a liquid crystal polymer resin, has recently attracted more attention than flexible circuit boards made of a polyimide. In a flexible circuit board made of a thermoplastic resin, accuracy of electrode shape and accuracy of the shape of the flexible circuit board made of a thermoplastic resin can easily be improved compared with a flexible circuit board made of a polyimide, and thus, consistently good electrical characteristics can be realized in a flexible circuit board made of a thermoplastic resin. In addition, such a flexible circuit board made of a thermoplastic resin has an advantage in that it does not have the above-mentioned disadvantage.
However, a flexible circuit board made of a thermoplastic resin has lower durability than that of a flexible circuit board made of a polyimide, and thus, in the case where deformation repeatedly occurs in a flexible circuit board made of a thermoplastic resin and in the case where deformation due to a large stress occurs in a flexible circuit board made of a thermoplastic resin, a failure such as fracture of the flexible circuit board made of a thermoplastic resin or breaking of wiring lines has sometimes occurred.